The present invention relates to process control systems and more particularly to means for controlling the concentration of a constituent in a product made by mixing components having differing amounts of the constituent.
Photographic emulsions can be prepared by mixing solutions of silver nitrate and alkali halide. The properties of such emulsions are, in part, a function of the defined shape, average size and size distribution of the silver halide crystals in the emulsion. To produce emulsions having desirable silver halide crystal characteristics, the silver ion concentration, acidity, flow patterns, temperature, and addition rate of the reactants must be closely controlled during the mixing operation.
One known system for controlling the addition rate of the reactants is a gravity feed system in which control valves are used to limit the rates of delivery of the silver nitrate and alkali halide solutions. In this system, a recorder is used to monitor acidity and silver ion concentration. Where a deviation from set point is sensed, the addition rates of the solutions are adjusted.
One disadvantage of such a system is that continuous variations in the primary addition rates of the reactants does nothing to enhance the overall stability of the process.
In another prior art system, the primary addition rates for the alkali halide and silver nitrate solutions are controlled by gear pumps in the primary flow paths for the solutions. The flow rate for each solution is monitored and controlled by a flow meter. Silver ion concentration is controlled in this system through the use of separate control flow paths from the reactant supply vessels to the mixing vessel.
Each control flow path includes a silver ion concentration analyzer, a gear pump and a flow meter. Where the silver ion concentration is below set point, the gear pump in the silver nitrate control flow path is energized. Conversely, where the silver ion concentration is above set point, the gear pump in the alkali halide control loop is energized. The control loops operate independently of the primary loop. Since the reactants addition rate is controlled by a gear pump, which is the only available means to adjust the flow rate, the limits to the range of motor speeds has a limiting effect on the turn-down ratio of the maximum to minimum addition rate through the control flow paths.